9/29/2004EE 42 fall 2004 lecture 131 Lecture #13 Power supplies, dependent sources, summary of ideal components Reading: Malvino chapter 3, 4.1-4.4 Next:

9/29/2004 EE 42 fall 2004 lecture 13 1

Lecture #13 Power supplies, dependent sources, summary of ideal components

Reading: Malvino chapter 3, 4.1-4.4

Next: 4.10, 5.1, 5.8

Then transistors (chapter 6 and 14)

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Topics

Today:

• Inductors and transformers

• Power supplies

• A new type of ideal device, the dependent source.

• Summary of ideal devices

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Inductors

• Any coil of wire will produce a magnetic field when current flows through it

• The magnetic field holds energy.• If the current is changed, the magnetic field will

change, and therefore the energy stored will change.

• Since the power must come from the circuit, this creates a voltage:

dt

dILV

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Symbol for and ideal inductor

• The symbol for an inductor is similar to that of a resistor, but with loops

dt

dILV

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Transformers

• A transformer is a couple of coils of wire which transfer power from one to the other by a changing magnetic field.

• By having different numbers of windings, or turns of wire, a transformer can step up or step down an AC voltage.

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Transformers

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Transformer symbols

• The symbol for a transformer is a pair of the same loopy lines used for inductors, but close together. If the inductor has a core of a magnetic material, it is shown as a couple of lines between the coils.

The number of turns in the coils willBe written nearby.

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Ideal transformer

• The voltage across the secondary of the transformer (the output windings) is:

• But this only works for changes in the voltage—and therefore for AC only

11

22 V

N

NV

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Ideal transformer

• The transfer of power can increase or decrease the voltage, and the current changes as well.

• Remember:

so if it is ideal (lossless)) 11VIP

2211 VIVIP

2

1

2

1

N

N

V

V

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Half-wave rectifier

• A single diode can be used to take an alternating current, and allow only the positive voltage swing to be applied to the load

~ R

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An AC input is sinusoidal

-1.5

-1

-0.5

0

0.5

1

1.5

0 5 10 15 20

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The diode blocks the negative voltages

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Full-wave rectifier

• If we add an additional diode, it does not pass current at the same time as the first diode, but the load is now disconnected during the negative half cycle.

• What if we could flip the connection and use the negative half wave?

~ R

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Full-wave rectifier

• The result is called a full wave rectifier

~ R

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Full-wave rectified voltage

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Filtering

• A transformer and a full wave rectifier will produce a voltage which is always positive, but varies with time

• In order to power electronic devices, we need to smooth out the variations with time.

• Another way to look at this is that we need to store energy temporarily while the input voltage changes sign.

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Power supply filter capacitor

• If we add a capacitor in parallel with the load, it will charge up when power is available from the voltage source, and then it will slowly discharge through the load when the diodes are off.

~ R

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Full wave rectified, with filtering

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Ripple

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

The result is a DC voltage, with some residual variations at twice the

frequency of the AC power. The variation is called ripple.

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Regulated power supply

• In order to produce a power supply without ripple, we will need to use an active device called a regulator

~ RConstant voltagepower supplyto load

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Ideal devices

Wire:Current in =current outNo voltage differences

Resistor

IRV

dt

dVCI

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Ideal devices 2

Inductor:

dt

dILV

Ideal diode:Reversed bias no current, open circuitForward bias no voltage drop, just like a wire

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Ideal devices 3

Transformer

2

1

2

1

N

N

V

V

+

V1

-

+

V2

-

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Ideal devices 4

~Voltage source:Voltage given, current can be anything

Note: the voltage could be given as A function of time

)(tV

Current sourceCurrent given, voltage can be anything

Note: the current could be given as A function of time

)(tI

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Ideal devices 5

~Dependent Voltage source:Voltage given as a multiple of another Voltage or a current, current can be anything

12 KVV

Dependent Current sourceCurrent given as a multiple of a differentcurrent or voltage, voltage can be anything)(tI

+

V1

-

+

V2

-

+

V1

- 12 GVI